1. Field of the Invention
The present invention relates to an optical WDM (Wavelength Division Multiplexing) transmission system which performs communications between center terminals (transmission equipment at central office) and user terminals (transmission equipment at subscriber premises) by using optical WDM, and particularly to an optical WDM transmission system and a method for configuring the same capable of accommodating, in a system having different allowed losses between center terminals and user terminals, the greatest number of user terminals within the allowed losses.
2. Description of Related Art
Recently, intensive researches have been carried out into PDS (Passive Double Star) optical subscriber network systems to implement FTTH (Fiber To The Home) in many nations.
FIG. 1 shows a conventional PDS optical subscriber network system, a partially modified one of the system disclosed in Japanese patent application laying-open No. 62-290219. The PDS system is provided with optical star couplers 7, which are passive components, at splitting points on transmission lines, and performs time division multiplexing bidirectional communications between center terminals 5 installed in a central office and a plurality of user terminals 6. This system makes it possible to reduce its cost by sharing the center terminals 5 and transmission lines among a plurality of user terminals 6.
The number of the user terminals 6 that can be connected to an optical star coupler is not fixed. For example, the user terminals in a subscriber area (A.sub.4, for example) at a short distance from the center terminal 5 are connected to the center terminal 5 through an optical star coupler 7 with a large splitting number. As the distance from the center terminal 5 increases, optical star couplers with smaller splitting numbers are used. For example, the user terminals in the farthest area A.sub.2 are connected to the center terminal 5 through a star coupler 7 whose splitting number is two. This is because losses from the center terminal 5 to the user terminals 6 increase with the transmission distance, and hence the splitting numbers must be reduced with the distance.
Let us suppose a PDS system that provides different types of services. Such a system handles various types of signals from low-rate narrowband signals like telephone signals to high-rate wideband signals represented by video signals. Thus, it handles diverse rate, capacity and data modulation.
It would be possible to integrally provide various services for the entire users by using a wideband signal. Such a system configuration, however, must employ high-rate transmission devices even for users who needs only low-rate services such as telephone, and this will increase the system cost.
There is still another problem. When receiving video signals, for example, an economical system can be implemented by utilizing commercially available TV sets. This will require analog modulation such as AM modulation, or FM modulation. The wideband analog signals, however, will not be directly transmitted through optical transmission paths because they cannot be electrically multiplexed with digital signals without conversion. To implement such a system by using high-rate digital transmission lines, converters for converting digital signals into analog signals and vice versa are needed, and this will increase its cost.
In view of this, a system using WDM transmission is proposed. For example, a system providing a low-rate bidirectional communication service like telephone simultaneously with a wideband unidirectional transmission service like CATV can be implemented by wavelength division multiplexing low-rate bidirectional transmission at 1.3 .mu.m band, and wideband unidirectional transmission at 1.55 .mu.m band. In such a PDS system using the WDM, a plurality systems offering different services can share the transmission lines.
The PDS system has an advantage of being economical because one center terminal is shared with multiple user terminals. For example, the center terminal connected with the user terminals in the area A.sub.4 of FIG. 1 has the highest economical effect due to user multiplexing in the center terminals in FIG. 1. Thus, an increasing splitting number gives a more economical system. On the other hand, optical loss is directly proportional to an increase in the splitting number. Since the allowed optical losses of the systems offering various services are limited, the splitting numbers must be set within the allowed losses. The allowed losses depend not only on the transmission distance, but also on the types of services, and the transmission rate and modulation method of the services. For example, to implement a wideband communication service such as video signal distribution in addition to an existing low-rate communication service like a public telephone service by using WDM, the allowed losses from the center terminals offering these services to a particular user terminal will differ from each other. This will cause the following problem in a system that offers different services by using WDM.
FIG. 2 shows a system configured by combining through a WDM device two point-to-multipoint optical transmission systems which offer services independent of each other.
In this figure, n user terminals 30-1-30-n are each connected to two center terminals 21 and 22 through optical fiber transmission paths 13, an n-optical splitter 12 and an optical WDM device 17. The forward link optical signals with wavelengths of .lambda..sub.1 and .lambda..sub.2 output from the center terminals 21 and 22, respectively, undergo the TDM (Time Division Multiplexing) by the center terminals and the WDM by the optical WDM devices 17, split by the n-optical splitter 12, and transmitted to the user terminals 30-1-30-n. Each user terminal acquires the optical signals sent thereto from the WDM forward optical signals.
On the other hand, some user terminals which carry out bidirectional communications in the user terminals 30-1-30-n transmit optical signals at timings assigned by the TDM. These optical signals transmitted from the user terminals are generated by modulating light fed from a light source in the user terminals or light fed from the center terminals. These reverse link signals from the user terminals are passively multiplexed on the time axis by the n-optical splitter 12, split into the optical signals with wavelengths of .lambda..sub.1 and .lambda..sub.2 by the optical WDM device 17, and received by the center terminals 21 and 22.
With such an arrangement, the user terminals 30-1-30-n can each communicate with the center terminals 21 and 22 by selecting one of the wavelengths. Thus, when the center terminals offer different services, each user terminal can receive different services through the same transmission paths.
The point-to-multipoint optical transmission systems which offer independent services are optimally designed individually regarding factors such as modulation methods, devices used, transmitted signal bandwidth, reliability, and cost. As a result, the allowed losses for the optical fibers and splitters take values proper to respective systems, and the numbers of user terminals accommodated by individual systems, which depend on the splitting numbers, differ from each other. For this reason, in the conventional method which integrates two or more systems offering different types of services into one system by WDM, the allowed loss of the new system is set at the minimum allowed loss among the systems to be integrated.
This will be described by an example in which a system 1 and a system 2 shown in FIGS. 4A and 4B are integrated. The system 1 accommodates eight user terminals 31-1-31-8 through an 8-optical splitter 14, and the system 2 accommodates four user terminals 32-1-32-4 through 4-optical splitter 15. These systems cannot be integrated into one system having a configuration as shown in FIG. 2.
In this case, it is necessary to separate the system 1 as shown in FIG. 4C, and then integrate the two systems 1 and 2 as shown in FIG. 3. More specifically, the two systems must be integrated in the steps of first separating the system 1 in accordance with the system 2 with a lower allowed loss (splitting number), and then combining the two systems 1 and 2 using a WDM device 17. Here, user terminals 35-1-35-4 are arranged as shown in FIG. 9A. Thus, when a plurality of PDS systems are integrated using one or more WDM devices to share transmission paths including star couplers, center terminals must be installed in accordance with the minimum splitting number among the possible splitting numbers of the respective systems. This hinders the conventional system from fully utilizing the splitting numbers of the individual systems.
This is a basic problem when utilizing the PDS transmission architecture and the WDM devices at the same time. Such a problem will not occur when the allowed losses are nearly equal in respective systems. The allowed losses, however, seldom agree with each other because the transmission methods and multiplexing numbers are independently determined to fully implement the individual services to be offered. In addition, designing the individual systems such that their allowed losses agree with each other will offset the advantage of the systems employing the WDM that a new system to be integrated with an existing system can be designed independently of the existing system.